Use has long been made within the packaging industry of packages of a single-use nature (so-called single-use disposable packages) for packing and transporting products such as liquid foods. A very large group of these single-use disposable packages is produced from laminated packaging material based on an interjacent core layer of paper or paperboard and outer laminate layers of some thermosealable plastic possessing superior liquid barrier properties, normally such as low density polyethylene (LDPE).
Depending on what food product is to be packed, i.e. its composition and storage sensitivity, its shelf-life, additional laminate layers--or laminate layers of other types than LDPE--may be included in the laminate structure. Examples of such additional or other laminate layers may be material layers possessing superior gas barrier properties, such as an aluminium foil or a layer of polyamide or of copolymers of ethylene and vinyl alcohol. Certain food products, such as juice, moreover place more stringent requirements to the effect that the packaging material possess superior aroma barrier properties, i.e. prevent flavour deterioration as a result of non-polar flavour and aroma substances being absorbed from the packed product into the packaging material. At the same time as the package must afford the product the best possible product protection properties, production of such single-use packages must also be simple and rational in order to be economically viable.
Within the prior art technology, polyethylene terephthalate (PET) has often been proposed as a material possessing superior aroma barrier properties, suitable for the inside layer in a packaging container for direct contact with the packed product, as opposed to, for example, LDPE. PET possesses extremely good barrier properties against essential oils such as D-limonen and other non-polar flavour and nutrient substances in, for example, orange juice and is, therefore, a highly desirable material for this purpose. However, PET suffers from the major drawback in employment as the innermost laminate layer in a packaging container for direct contact with the packed product in that it is difficult to thermoseal at rational production speeds, in particular on sealing of the longitudinal joints in a packaging container produced from a sheet-shaped packaging laminate blank in which the longitudinal edges of the sheet-shaped blank overlap one another and are exposed such that the outside of the inner edge is sealed against the inside of the outer edge. In rational production of conventional packaging containers, such longitudinal joint sealing takes place at very high speeds, in that the sheet-shaped blanks in rapid sequence are advanced, reformed and longitudinally sealed by means of thermosealing into tubular packaging container blanks. The term "tubular" is hereafter taken to signify tubes of both circular and quadratic or rectangular cross section. For thermosealing of PET, it is necessary that the pressure from the sealing jaws is maintained during the heating process, at least up to approximately 165.degree. C., which takes roughly 0.5 sec. However, the available stay time during the sealing process on sealing of longitudinal joints, i.e. the time during which the pressure from the sealing jaws is maintained, is only approx. 0.01 sec., and thereby insufficient. On the other hand, sealing of the top and bottom of the same packaging containers takes place intermittently in connection with the product being filled into the container, which permits longer stay times in the sealing operation proper, and thereby makes for thermosealing by means of surface fusion between two PET layers.
Attempts have been made to overcome these difficulties in various manners, for example by employing a modified PET which facilitates thermosealing. From, for example, European Patent Application EP 0 237 235, it is known that glycol-modified PET, so-called PETG, may be thermosealed. However, a serious drawback inherent in this glycol-modified PET is that it results in a more brittle material layer with less flexibility and durability and is thus not as desirable in a packaging laminate as normal, amorphous, non glycol-modified PET. Moreover, nor can thermosealing take place using PETG at such high production speeds as are actually desirable.
A method of attempting to circumvent the difficulties in the heat sealing of packaging blanks with difficulty sealable inside lavers such as amorphous PET has been to seal the longitudinal overlap joints with hot melt glue, i.e. by applying a hot melt glue along the one edge in the form of a strand by means of a nozzle and then compress this edge with the other edge in the overlap joint between the two edges. However, it has not hitherto been possible to achieve uniform and operationally reliable application at rational sealing speeds with the aid of this technology. Major problems have been encountered with large quantities of waste hot melt glue and with the fact that the application of the hot melt glue becomes uneven and difficult to control at those small quantities which are required, with tacky outflows as a result In order that a hot melt glue be able to adhere to surfaces of PET, and surfaces with similar adhering properties as PET, it is necessary that the glue be extremely tacky, i.e. extreme adhesion or bonding forces against the surface intended for gluing, which in turn entails that the hot melt glue becomes difficult to handle on application, and that the control of the applied quantity and the problem of outflowing glue is aggravated. As a result of uneven application, the strength of the glued joints will also be uneven and consequently the tightness properties of the packaging container in the sealing joints are unreliable. Moreover, the high tacky or adhesive forces entail that the application speed is limited to unrational production speeds, for which reason hot melt glue sealing of packaging containers for liquid foods on an industrial scale hardly occurs on the market today. The outflow of the applied hot melt glue strand will also be uneven, since hot melt glue has not been applied at certain points in a sufficient quantity to cover the entire width of the gluing region and at other points has been applied in an excessive quantity and therefore flows out beyond the sealing region, which can result in visible, unsightly hot melt glue lumps outside the sealing joints and, in certain cases, that the excess hot melt glue comes into direct contact with the packed product.
An applicator nozzle continuously feeds a strand of hot melt glue, with the result that it is as good as impossible to apply a hot melt glue strand to sheet-shaped packaging blanks which pass the nozzle with interspace between them, but that the hot melt glue sticks and tacks to every area. The applicator nozzle cannot be shut off and turned on at the same rate, partly because the inertia in such a system with high viscosity hot melt glue renders it difficult to control the applied quantity at the beginning and at the end of the sheet, and partly since a nozzle for intermittent feeding more easily becomes blocked and otherwise causes operational disruption.
Another difficulty in the striving to produce a packaging container with superior aroma barrier properties is that such a hot melt glue sealed longitudinal joint is not durable in long-term cold storage. Thus, such a packaging container has not displayed durable shelf life for a lengthy period of time with superior liquid, gas and aroma barrier properties, because of the fact that the hot melt glue sealed longitudinal joint has become untight.
In respect of liquid-, gas-, and aroma barrier properties, such incision edges of the sheet-shaped packaging blank which are freely exposed to the packed product create problems in that gas and liquid molecules, like non-polar flavour substances, are slowly absorbed in the packaging material through the thus freely exposed incision edges.
Thus, within the prior art technology, it has not hitherto been possible, in a cost effective and rational manner, to produce packaging containers from substantially planar packaging blanks with inside and sealing layers with properties similar to PET as regards thermosealing properties and adhesion to hot melt glue.
Nor has it hitherto been possible in a cost effective and rational manner using a hot melt glue to longitudinally joint seal packaging containers from packaging blanks with inside layers of non-thermosealable or difficultly thermosealable plastics, such as, for example, PET, this inside layer moreover displaying poor adhesion properties vis-a-vis conventional hot melt glue.